This invention relates to measuring electrical currents.
A current is typically measured by passing it through a known resistance and measuring the resulting voltage difference between the two ends of the resistance. The ability of the measuring circuit to accurately respond to the differential voltage while ignoring the common mode voltage (the average of the voltages at the two points, which may be many times larger than the differential voltage) is called the common mode rejection.
A typical measuring circuit receives as inputs the voltages at the two measuring points and delivers as its output a single voltage (referenced to a ground) that is an amplified version of the differential voltage with the common mode voltage component eliminated.
In instrumentation amplifier type measuring circuits, each of the input voltages is amplified separately and the amplified versions are then subtracted from one another both to eliminate the common mode voltage and to derive an amplified version of the differential voltage.
Isolation amplifier type measuring circuits have an inductive, optical, or capacitive barrier. Only the differential voltage can cross the barrier and appear at the output. The common mode voltage appears between the lower one of the input voltage and the ground reference of the output voltage. The differential voltage is passed across the barrier by causing a voltage or current on the output side of the barrier to track exactly a voltage or current on the input side which is arranged to be representative of the differential voltage of interest.